1. Field of the Invention
Applicant's invention relates to the high speed transmission of electrical information over a bandwidth limited media.
2. Background Information
Rapid information transmission is achieved by using a signal that changes rapidly with time. Because all real electrical systems contain stored energy however, a change in the signal cannot be accomplished without a corresponding change in the stored energy of the system. A definite amount of time is required to change the stored energy of any real system. Therefore, the system's ability to respond to a change in the signal is limited. If the rate at which the signal varies is arbitrarily increased, the system will eventually be unable to respond to the signal changes and the information will not be transmitted.
A signal's bandwidth is one measure of its speed. Similarly, the system's bandwidth represents its ability to respond to signal changes. Transmitting a large amount of information in a small amount of time requires wideband signals to represent the information and wideband systems to accommodate the signals. If the system's bandwidth is insufficient, it may be necessary to decrease signaling speed and thereby increase transmission time. Frequency modulation (FM) is a common technique used to communicate data over a bandwidth limited transmission channel. In FM systems, the changing values of the information signal are reflected in corresponding changes in the carrier signal's frequency. An FM demodulator must therefore determine the frequency of the carrier signal and translate the frequency back into a signal value. The design of conventional demodulators employing frequency locked loops, phase locked loops, and integrators driven by zero crossings detectors limit the signal bandwidth to a percentage of the carrier bandwidth. Conventional demodulators also tend to require several signal cycles to resolve a particular frequency. These constraints place limitations upon the data transmission rate, f.sub.d, in addition to those imposed by the transmission media bandwidth.
One well known application for transmitting date using FM is referred to as Frequency Shift Keying (FSK). Data transmission by FSK assigns a specific carrier frequency to a particular data state. If there are two data states, i.e. one bit of date per state, then there are two frequencies used. If there are eight data states (three bits of data), then there are eight frequencies used. The most common implementation of FSK transmits one bit of data per data state, each state being represented by one of two frequencies. These frequencies are normally referred to as the "mark" frequency, f.sub.m, and the "space" frequency, f.sub.s.
Demodulation considerations generally dictate that the FSK signal modulation index, I.sub.mod, be greater than or equal to 0.5 where I.sub.mod =f.sub.dev /f.sub.d and f.sub.dev, the frequency deviation is the difference, in Hertz, between the mark frequency and the space frequency and f.sub.d, the data rate in bits/second. This limitation on I.sub.mod exists primarily because frequency shifting results in a pair of uncorrelated sin(x)/x frequency spectra centered about each of the transmission frequencies. A typical spectrum of a phase continuous toggled FSK signal is shown in FIG. 1 for a signal with a deviation of 4 MHz and a transmission rate of 1 Mbits/second. Matched demodulation filters must recognize the frequencies and the frequency separation enables each filter to resolve the power difference between a mark and a space.
Reduction of the modulation index to 0.5 produces Minimal Shift Keying (MSK). The frequencies used for MSK can be resolved in a linear system using matched filters. Referring back to FIG. 1, it can be seen that the two main power peaks are 2 MHz away from the center frequency. The shift rate of 1 MHz has produced side lobes on both of the major lobes that are 1 MHz away from the main lobes. As the data transmission rate, f.sub.d, increases, the side lobes from the two main lobes move closer to the opposite main lobe. If the data rate exceeds the deviation, resolving the mark and space frequencies with a filter becomes difficult.
Phase continuous toggling between the two transmitted frequencies at a rate greater than the frequency difference between the mark and space frequencies, i.e. at I.sub.mod &lt;0.5, is Subminimal Shift Keying (SSK). The present invention represents the first synchronous application of grossly undermodulated phase continuous FM to transmit information. While systems have been suggested in which the instantaneous frequency of the carrier wave changes every half-cycle of the carrier waveform, the data rate of the present invention is independent of the data content, eliminating the need for FIFO buffers. Moreover, the present invention does not require that any fixed percentage of the signal cycle be modulated onto the carrier signal. In other words, the present invention is capable of retrieving information from a signal that changes instantaneous frequency more often than once per half cycle. Thus, higher data rates, as a percentage of the signal bandwidth, are achievable in the present device. Finally, the present invention does not rely upon zero crossing detectors for its timing. This eliminates the need for comparator circuits which are inherently prone to baseline wander and other small shifts in signal amplitude levels.
SSK modulation creates overlapping frequency spectra that precludes recovery in a linear system using matched filters. For example, switching between a space frequency of 13 MHz and a mark frequency of 17 MHz at a 12.5 MHz rate (i.e., every 40 nsec or, at 25 Mbps) results in the spectra shown in FIG. 2. The bandwidth is approximately EQU BW=2f.sub.d +f.sub.dev (Eq. 1)
The center frequency of the spectra will be the arithmetic mean between the mark and space frequency, f.sub.c =(f.sub.m +f.sub.s)/2 where f.sub.m =the mark frequency and f.sub.s =the space frequency. In the example, the center frequency is 15 MHz. Modulating the SSK generator with a random signal (i.e. true data) as shown in FIG. 3 yields a spectrum bounded by approximately the bandwidth predicted by Equation 1.
The method being suggested for recovery of the SSK data is to multiply the baseband frequency spectrum. Multiplication of the frequency spectrum of an FM signal multiplies the deviation frequency, f.sub.dev, but does not multiply the limits of the spectrum established by the data rate, f.sub.d, so that the multiplied spectrum bandwidth is EQU BW.sub.mult =2f.sub.d +mf.sub.dev (Eq. 2)
where m=the order of the frequency multiplication. For example, doubling the frequency of the example spectrum three times results in a bandwidth of 41 MHz at a center frequency of mf.sub.c or 120 MHz. By increasing f.sub.dev while holding f.sub.d constant, the multiplication process improves I.sub.mod so that the multiplied signal can be demodulated in a conventional frequency discriminator.
Multiple frequencies can be used in an SSK system to represent multiple data states. The more frequencies used in the limited frequency shift region, the more times the signal will have to be multiplied to facilitate data recovery. An implementation of the preferred embodiment with a bit rate of 125 Mbps uses 5 bits per symbol (or 2.sup.5 =32 distinct frequencies) and 3 voltage squaring circuits.
The frequency deviation and center are controlled by routing serial to parallel 10 outputs to appropriate inputs of phase accumulator 18. For example, to set the center frequency at 15 MHz with about +/-1.5 MHz of deviation on a system with N=5 and K=32 and system clock 24 operating at 75 MHz, delta phase symbol 12 must range from a value equivalent to 1.1310 radians to a value equivalent to 1.3823 radians. With N=5 and K=32, 27 inputs to phase accumulator 18 will remain static while the 5 remaining bits range from 00000 to 11111. The 27 static inputs to are set to obtain the initial value of 1.1310 and the 5 active lines are tied to the appropriate inputs such that as the 5 lines range from 00000 to 11111, phase symbol 12 will range from the initial value to a value equivalent to 1.3823.
Using a 125 Mbps bit rate divided by N=5 gives a data rate, f.sub.d, of 12.5.times.10.sup.6 symbols/sec. The frequency deviation, f.sub.dev, of 3 MHz gives a I.sub.mod of approximately 0.12 for the transmitted signal. Upon passing through three voltage squaring circuits, I.sub.mod will be on the order of 0.96 which is more than sufficient to permit data recovery.
Although the preferred embodiment implements the SSK multi frequency oscillator with a direct digital synthesizer consisting of phase accumulator 18, look-up table ROM 20, and digital to analog converter 22, other configurations are possible. As long as the modulator has the capacity to generate a phase continuous signal with a modulation index significantly less than 0.5, specifics of the implementation are irrelevant. Similarly, the use of a voltage squarer circuit 44 in Demodulator 42 is merely a possible implementation. In a system employing a frequency shift in transmitter/conditioner 30, a conventional frequency doubler may be substituted for the voltage squarer circuit 44.